Holograms have been used primarily in the fields of stereo image display and security. The materials mainly used for holograms in old times were wet-type photosensitive materials that required a process for development, for example, silver salt and dichromated gelatin. Recently, however, dry-type photopolymerizable hologram recording materials that require no process for development and show excellent environment resistance and light resistance after recording have become the mainstream. The mechanism for recording by a photopolymerizable hologram recording material differs from that for recording by a silver salt photosensitive material or dichromated gelatin and is generally considered as follows. When a recording material is irradiated with an interference pattern formed by interference of light of highly coherent nature (capable of displaying stable interference effects), the substances existing in the material and differing from one another in polymerization ability diffuse and migrate respectively to a bright part of the light (area of higher light intensity) and a dark part of the light (area of lower light intensity) to form a refractive index-modulated structure as the photopolymerization progresses, thereby recording a hologram. There are three modes of polymerization for the photopolymerizable hologram recording materials: (1) radical polymerization, (2) cationic polymerization, and (3) radical/cationic polymerization. Materials of the photoradical polymerization type are commonly used; however, they have a serious defect in that the shrinkage attributable to this particular mode of polymerization amounts to as high as several percents in spite of their high sensitivity.
In recent years, as a part of various activities organized toward realization of a ubiquitous information society, studies on the use of photopolymerizable hologram recording materials for holographic data storage have been renewed throughout the world and a variety of photopolymerizable hologram recording materials have been proposed. This situation is largely due to enormous progresses made in technologies relating to laser optical sources, spatial light modulators, and the like in the execution of the federal projects of the United States initiated in the mid-1990s such as HDSS (Holographic Data Storage System, 1995-1999) and PRISM (Photorefractive Information Storage Materials, 1994-1998). The conventional technology for recording data on an optical disk such as a CD and a DVD comprises condensing laser beams on a recording material through a lens and recording data on the material bit by bit. The data storage capacity that depends on the size of a spot of condensed beams has already reached the theoretical limit and a further increase in storage capacity would require novel technologies. It is holography that has attracted attention in recent years as one of the candidates for such technologies.
The holographic data storage can perform volume recording unlike plane recording in the case of CDs and DVDs and treats page data. For this reason, the holographic data storage can record a vast amount of data, far surpassing the conventional CDs and DVDs. Currently, developmental works on materials and recording technologies are in progress actively at research organizations aiming at putting to practical use of hologram optical disks of the WORM (Write Once Read Many) type that is capable of performing 1 TB (terabyte) recording. The concept for the basic design of materials for the holographic data storage is the same as for the aforementioned photopolymerizable hologram recording materials for use in stereo image recording and security. However, the materials for holographic data storage must satisfy extremely stringent requirements, in particular, the requirements for sensitivity to a light source to be used and cure shrinkage.
As described in patent documents 1 and 2, materials with controlled polymerization shrinkage are prepared by using CROP (Cationic Ring-Opening Polymerization) monomers having oxirane and oxetane rings and pre-photoirradiating them. In this case, cationic polymerization is affected less by oxygen inhibition than radical polymerization, but the sensitivity in a long-wavelength region somewhat deteriorates. On the other hand, patent document 3, 4, or 5 discloses the combined use of cyclohexene oxide and an expanding agent (a diphenylfurancarboxylic acid salt) in the preparation of a material whose polymerization shrinkage is as low as 1% or less. Although the cure shrinkage is suppressed here also by the use of a CROP monomer followed by whole-area exposure of the monomer before or after recording, no proposal is made on improvement of the sensitivity. Putting the holographic data storage to practical use would require a further increase in photosensitivity, a reduction in cure shrinkage, and an increase in heat resistance                Patent document 1: U.S. Pat. No. 5,759,721        Patent document 2: U.S. Pat. No. 6,784,300        Patent document 3: U.S. Pat. No. 3,993,485        Patent document 4: U.S. Pat. No. 6,124,076        Patent document 5: JP2000-086914 A        Patent document 6: JP5-94014 A        Patent document 7: JP9-106242 A        Patent document 8: JP2004-123873 A        Non-patent document 1: Macromolecules, 34, 396-401 (2001)        
Patent documents 6 and 7 describe a photosensitive resin composition for hologram recording formulated from a radically polymerizable ethylenic monomer, a photopolymerization initiator, and an epoxy resin. This composition utilizes the following phenomenon for forming a hologram; the radically polymerizable ethylenic monomer starts polymerizing preferentially from the bright part of an interference pattern at the time of exposure to laser beams and the ethylenic monomer migrates to the bright part. The epoxy resin is cured thereafter. Patent document 8 describes a soluble aromatic copolymer having a structural unit of a divinyl aromatic compound and a structural unit of a monovinyl aromatic compound, but contains no description on the use of the copolymer in holograms.